Coating process

ABSTRACT

This invention provides a method for producing uniform or nonuniform coatings of aluminum intermetallics on articles consisting of nickel- or cobalt-base alloys, using a reduced pressure pack coating technique in which the coating is derived from an alloy containing at least 40 percent by weight of aluminum.

United States Patent 72] Inventors Sanford Baranow Woodbrldge;

William R. Freeman, Jr., Easton, both 01 Conn.

References Cited Primary Examiner-Alfred L. Leavitt Assistant Examiner Kenneth P. Glynn Anomeys-Char1es M. Hogan, Irwin P. Garfinkle and Lawrence 1. Field ABSTRACT: This invention provides a method for producing uniform or nonuniform coatings of aluminum intermetallics on articles consisting of nickelor cobalt-base alloys, using a reduced pressure pack coating technique in which the coating is derived from an alloy containing at least 40 percent by weight of aluminum.

Appl. No. 1,750 Filed Jan. 9, 1970 Patented Dec. 7, 1971 Assignee Avco Corporation Stratford, Conn.

COATING PROCESS 9 Claims, 9 Drawing Figs.

US. Cl. l l7/107.2 P, 148/63 Int. Cl ..C23c 11/00, C23c 13/00, C23c 17/02 Field oiSearch 117/1072 R. 107.2 P

B w s INVENTORS ATTORNEY Son ford BOrdnow Wi Ilia m R, Freeman,Jr

SHEET 1 OF 2 FIG.

PATENTED DEC 7 197i w J 11- u PATENIEU DEC 7 Ian 'SHEET 2 BF 2 FIG. 6.

FIG.

FIG. 8.

INVENTORS Sanford Bara now William R.Freemon,Jr.

ATTORNEY COATING PROCESS This invention relates to pack aluminizing articles composed of nickelor cobalt-base superalloys, in order to provide the articles with an outer coating of NiAl or CoAl. By suitably adapting the process, it is also possible to obtain either coatings of uniform thickness or coatings of nonuniform thickness in which the thickness may taper in a desired pattern as hereinafter described.

The invention will be more fully understood from the description which follows taken in conjunction with the drawings in which FIG. 1 is a schematic plan view showing one manner of loading of a furnace unit to produce a desired tapered thickness in the coating of a turbine blade, taken on a plane corresponding to BB in FIGS. 2, 3 or 4;

FIGS. 2, 3 and 4 are schematic views showing other arrangements for obtaining nonuniform coatings on the processed articles, taken on a plane corresponding to A-A of FIG. 1;

FIG. 5 is a view like FIG. 1 showing a loading pattern wherein a more uniform coating is obtained;

FIG. 6 is a schematic view in section of a turbine blade coated by the process of this invention;

FIG. 7 is a fragmentary view, greatly magnified, schematically depicting a coated product produced by the process of this invention; and

FIGS. 8 and 9 are schematic plan and elevation views of a vacuum furnace arrangement, partly broken away.

The furnace shown schematically in FIGS. 8 and 9 comprises an enclosure 10 provided with heating means (not shown), which means are readily controllable in known manner to produce any desired temperature in the furnace and to maintain the furnace and its contents at that temperature. Extending through a wall 12 of enclosure 10 is a conduit 14 which connects the enclosure with a source of vacuum shown here as a diffusion pump 16 and a mechanical pump 18 connected in series through conduit 20. Another conduit 22 connects the inside of enclosure 10 to a source of argon or other inert gas, stored in a reservoir 24. The enclosure of heating chamber 10 is provided with a hearth 30 on which there may be positioned one or more containers 32 such as those shown in FIGS. 1 to 5 as cans of nickel or steel open at the top and adapted to be covered by a loosely fitting cover 34 having one or more perforations 36 therein to permit free passage of gases between the heating enclosure 10 and the individual containers 32 and to permit the contents of the individual containers to be subjected to pressures lower than atmospheric, when the furnace 10 is evacuated, or to atmospheres of desired composition introduced into the furnace through conduit 22.

As shown in FIGS. 2, 3 and 4, the containers 32 and 32 and the disposition of the articles being aluminized may be varied to produce nonuniformity in the thickness of the aluminum coating deposited thereon. For example, a thickness variation may be achieved by distributing the articles radially around a metal rod or arbor 40 as shown in FIG. 1. Arbor 40 may be welded to the bottom of container 32. When the enclosure 10 is heated, heat flows toward the articles through the walls of the containers 32, 32' as shown by arrows 42 and additional heat flows toward the articles from the centrally disposed metal arbor 40, 40'. The temperature of any point on the surface of the article depends on the temperature within the heating enclosure 10, the apparent thermal diffusivity of pack material (chiefly a function of furnace pressure), and also depends on the distance from the point on the surface of article to the surface of container 32 and to the surface of arbor 40. The arrangement in FIG. 1 has been found to be very desirable for coating turbine blades, even those as small as one-half inch in width, the leading edges of which require a heavier coating than the trailing edges. The leading edges are placed nearest to the wall of the retort 32 and the trailing edges are disposed adjacent to the metal arbor 40. As a result of the difference in temperature along the surface of the articles, accentuated by the use of a vacuum which results in a low heat transfer rate through the coating pack, more coating material deposits on the warmer leading edges than deposits on the cooler trailing edge, because a thicker coating deposits on the wannest surface of the article and the coating which deposits on the coolest surface of the article is thinner, the total amount of the deposit at any point depending on both the temperature and the time interval that point on the article surface is at coating depositing conditions.

FIG. 6 shows a turbine blade on which the coating varied from a minimum of 1.75 mils. atone end to a maximum 3.5 mils. at the other end, due to the variation in the time-temperature relationship, the blade measuring 0.75 inches from the leading edge to the trailing edge.

In FIG. 2, rod 40' is shown tapered from a thicker top to a thinner bottom, in order to produce a heating pattern varying axially along the container, the taper being exaggerated for purpose of illustration.

Other means and other loading patterns for controlling the heat transferred to articles being coated within the furnace will be readily apparent to persons skilled in the art. For example, in FIG. 3, the articles are packed at distances varying from the inner wall of can 32 and in FIG. 4, the container 32 is tapered rather than cylindrical.

The diffusion coating process is performed in a moderate vacuum in unsealed containers utilizing as the source of the aluminum in the deposited coating, a commercially available alloy preferably consisting of 56 percent Cr and 44 percent Al, by weight. Alloys having less than about 40 percent by weight of aluminum do not produce a good NiAl or CoAl deposit and those having too high an aluminum content melt at temperatures below the temperature limit for this coating process. Consequently, a minimum of about 40 percent aluminum and a maximum of about 48 percent aluminum represents a somewhat critical range for the starting material. The Cr-Al alloy is crushed to a mesh size between 50 and +325 (Tyler standard sieve) and then is mixed with about one half its weight of Al 0 powder of about the same size. The mixture is poured into cans 32 and the articles 50 to be coated are placed on the bed of the mixture and then covered with additional mixture. The reason for the use of 50 and +325 mesh powder will be explained later.

It appears that the use of an alloy with a high aluminum content and a melting point well above the temperature at which it decomposes when the alloy is heated in a vacuum along with the use of a lower pressure during the soaking step described below results in vaporization of some of the aluminum from the alloy powder and deposition of the vapor onto the articles embedded in the pack material. Competition always exists between backstreaming furnace contaminants (chiefly water vapor) and the aluminum vapor so that thinner, aluminum poor coatings are derived when coarser and, therefore, less surface active powders are utilized. Finer than 325 mesh powders may be used, but constitute a health hazard. It will be noted that no activator or halide salt, such as ammonium chloride commonly used in this art, is utilized in the present process. It has been found, however, that when a very small amount of ammonium chloride is included in the coating batch composition it functions to flush out any residual air from the coating pack during the initial heating of the pack during its vaporization and that it also increases the rate of heat transfer during the early stages in which the pack is heated up from room temperature to about I ,600 F.

By interrupting the heating after the pack has been heated up to about l,600 F., it has been established that all traces of the ammonium chloride have disappeared therefrom before any coating has taken place and that no halide remains to act as a carrier during the next step in the coating process.

An example of a heating cycle involves heating first to 1,600 F. at a rate of roughly I,200 F. per hour while subjecting the heating furnace to a subatmospheric pressure by use of pumps I6 and I8; holding the pack at 1,600 P. for 2% hours at a pressure of 10 microns or less, whereby adsorbed air or other contaminating gas are eliminated from the pack; and

then heating to l,900 F. in about 15 minutes and holding at this temperature for 5 to 10 hours. Thereafter, the pack is cooled to l,200 F. while the vacuum is maintained. When the pack reaches l,200 F., the vacuum line 14 is closed and argon or other inert gas is admitted to the enclosure through line 22, whereby the pack and its contents are rapidly cooled to a temperature at which the coating is no longer affected by the atmosphere.

We claim:

1. A method for diffusion coating metallic articles consisting essentially of a superalloy based on a metal selected from the group consisting of nickel and cobalt, which method comprises:

embedding said articles to be coated in a particulate charge material comprising particles between minus 50 and plus 325 Tyler Standard Sieve particle size and containing particles of an alloy charge material containing at least about 40 percent Al and the balance substantially all metal selected from the group consisting of Cr, Fe, Ni and Co, plus incidental impurities; and

heating said charge material and the articles contained therein to a temperature of at least about l,600 F. at a heating rate of between l,000 and 1,200 F. per hour, and at a pressure not exceeding 10 microns, and thereafter the charge mixture and embedded particles are held at about l,600 F. for about 2% hours and then heated to a temperature of abut'l,900 F., at a heating rate of between 1,000 and L200? F. per hour, while main taining the same at a pressure not exceeding 1 micron abs., and holding the'charge and contained articles at this temperature for between about 5 and about hours.

2. The process of claim 12 wherein the process is carried out with a charge material comprising an alloy of 56percent Cr plus 44% Al by weight.

3. The process of claim 2 wherein the charge material consists of about 2 parts by weight of metallic alloy and 1 part by weight of particles of an inert metal oxide.

4. The process of claim 3 wherein the inert metaloxide is M 0 5. The process of claim 1 wherein, after heating for a time sufficient to form the desired coating on said articles, the charge mixture and embedded articles are vacuum cooled to about 70 F.

6. The process of claim 1 wherein, after heating for a time sufficient to form the desired coating on said articles, the charge mixture and embedded contents are exposed to a cold inert gas and thereby cooled to about 70 F.

7. A method for diffusion coating metallic articles having an outer surface consisting essentially of a superalloy based on a metal selected from the group consisting of nickel and cobalt, which method is characterized by the production of coatings of nonuniform thickness on said articles, said method comprising:

embedding said article to be coated in particles of an alloy charge material containing at least about 40% Al and the balance substantially all metal selected from the group consisting of Cr, Fe, Ni and Co, plus incidental impurities, said articles being oriented and arranged on a definite pattern in said charge material; providing at least one source of heat for heating said charge materials and the articles embedded therein; and heating said charge material and the articles contained therein to a temperature of at least about l,600 F, said heating being in the absence of any carrier material and under a pressure substantially below normal atmospheric pressure and in the absence of any oxidizing atmosphere, for a time sufficient to form the desired coating on said articles, and said heating being effected in a manner which produces a nonuniform temperature on the surfaces of said embedded articles so that there is at least a first surface portion at a temperature higher than the average temperature to which said article is heated and at least a second surface portion at a temperature which is less than the average surface temperature to which said article is heated, the difference in temperature between said first portion and said second portion being sufficient to produce a difference in the thickness of coating formed on said articles during the time interval they are subjected to said heating.

8. The method of claim 7 wherein the nonuniform temperature on the surface of said articles is produced as a result of the manner in which said articles are embedded in said pack.

9. The method of claim 7 in which the nonuniform temperature on the surface of said articles includes first surface portions of said articles embedded in said charge so as to be disposed in relatively warmer regions of the charge mixture and second surface portions of said articles embedded in said charge so as to be disposed in relatively cooler regions of said charge mixture, whereby a thicker coating deposits on said first surface portions than the thinner coating deposited on said second surface portions.

"unto n-ioA 

2. The process of claim 12 wherein the process is carried out with a charge material comprising an alloy of 56percent Cr plus 44% A1 by weight.
 3. The process of claim 2 wherein the charge material consists of about 2 parts by weight of metallic alloy and 1 part by weight of particles of an inert metal oxide.
 4. The process of claim 3 wherein the inert metal oxide is Al203.
 5. The process of claim 1 wherein, after heating for a time sufficient to form the desired coating on said artiCles, the charge mixture and embedded articles are vacuum cooled to about 70* F.
 6. The process of claim 1 wherein, after heating for a time sufficient to form the desired coating on said articles, the charge mixture and embedded contents are exposed to a cold inert gas and thereby cooled to about 70* F.
 7. A method for diffusion coating metallic articles having an outer surface consisting essentially of a superalloy based on a metal selected from the group consisting of nickel and cobalt, which method is characterized by the production of coatings of nonuniform thickness on said articles, said method comprising: embedding said article to be coated in particles of an alloy charge material containing at least about 40% A1 and the balance substantially all metal selected from the group consisting of Cr, Fe, Ni and Co, plus incidental impurities, said articles being oriented and arranged on a definite pattern in said charge material; providing at least one source of heat for heating said charge materials and the articles embedded therein; and heating said charge material and the articles contained therein to a temperature of at least about 1,600* F, said heating being in the absence of any carrier material and under a pressure substantially below normal atmospheric pressure and in the absence of any oxidizing atmosphere, for a time sufficient to form the desired coating on said articles, and said heating being effected in a manner which produces a nonuniform temperature on the surfaces of said embedded articles so that there is at least a first surface portion at a temperature higher than the average temperature to which said article is heated and at least a second surface portion at a temperature which is less than the average surface temperature to which said article is heated, the difference in temperature between said first portion and said second portion being sufficient to produce a difference in the thickness of coating formed on said articles during the time interval they are subjected to said heating.
 8. The method of claim 7 wherein the nonuniform temperature on the surface of said articles is produced as a result of the manner in which said articles are embedded in said pack.
 9. The method of claim 7 in which the nonuniform temperature on the surface of said articles includes first surface portions of said articles embedded in said charge so as to be disposed in relatively warmer regions of the charge mixture and second surface portions of said articles embedded in said charge so as to be disposed in relatively cooler regions of said charge mixture, whereby a thicker coating deposits on said first surface portions than the thinner coating deposited on said second surface portions. 